Income Taxes - Summary of Difference in Effective Tax Rates and Statutory Federal Income Tax Rate of 35% (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Tax at statutory rate, amount	$ 2,467	$ 3,718	$ 7,108	$ 8,499
Tax exempt income, amount	(85)	(164)	(266)	(499)
Life insurance, amount	(92)	(148)	(273)	(416)
Stock compensation, amount	(12)	(102)	(144)	(300)
Other, amount	(61)	(237)	(205)	(283)
Income tax provision, total amount	$ 2,217	$ 3,067	$ 6,220	$ 7,001
Tax at statutory rate, rate	21.00%	35.00%	21.00%	35.00%
Tax exempt income, rate	(0.73%)	(1.54%)	(0.79%)	(2.05%)
Life insurance, rate	(0.78%)	(1.39%)	(0.81%)	(1.71%)
Stock compensation, rate	(0.10%)	(0.96%)	(0.42%)	(1.24%)
Other, rate	(0.52%)	(2.23%)	(0.60%)	(1.17%)
Income tax provision, total rate	18.87%	28.88%	18.38%	28.83%